1. Field of the Invention
This invention relates to a color image forming apparatus using the electrophotography, and more particularly to a color image forming apparatus using an image retainer and a transfer member in which a copy mode can be changed to one of a multicolor mode and a monochromatic mode.
2. Description of the Prior Art
As a color image forming apparatus, an apparatus as disclosed in the official Gazette on Japanese Patent Laid-Open No. 76766/1985, for example, has been proposed. In such apparatus, a color toner image is formed on an image retainer by repeating writing of each color information by a laser beam scanner and developing with developers each including different color toner, transferred at a time electrostatically on a transfer material by using a corona discharger, and then fixed to form a color image. Such apparatus has a high precision of superposition of the different color toner images, because toner images different in color are superposed on the image forming member. However, it has a problem that a toner image formed previously affects on a toner image formed thereafter. Further, a good color image can not be obtained by the conventional analogue system.
Accordingly, the Official Gazetter on Japanese Patent Laid-Open No. 123257/1986 suggests such a technique that a rotary transfer drum is used and different color toner images are superposed on a transfer material wound around said rotary transfer drum. In this technique, a color toner image is formed on the transfer material by repeating a series of steps of writing on the image retainer with a laser beam modulated by a digital signal of separated color image informations, developing with a developer containing a color toner, and transferring a color toner image obtained by said developing on a transfer material wound around the transfer drum, and by fixing a color toner image thus obtained on the transfer material. Specifically, this technique is similar to the black and white image forming process wherein toner images are transferred on the transfer material whenever one toner image is formed on the image forming drum.
Further, there has been proposed an apparatus wherein color toner images different in color are superposed on a transfer drum by repeating said series of steps on the transfer drum, and the superposed different color toner images are transferred electrostatically at a time on a transfer material, and then fixed to form a color image.
It is necessary to use a transfer drum of which peripheral length is longer than the length of a recording paper on which the color image recording is carried out, even in a case that toner images are superposed on a transfer material wound around a transfer drum, or that toner images are superposed on the transfer drum, as mentioned above. Accordingly, a color image forming apparatus using an image forming member and a transfer member for obtaining a color image of a large size becomes large in size. On the other hand, it is required sometimes to obtain an image of monochromatic by said color image forming apparatus.